Electrolytic capacitors (e.g., tantalum capacitors) are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. For example, one type of capacitor that has been developed is a solid electrolytic capacitor that includes an anode (e.g., tantalum), a dielectric oxide film (e.g., tantalum pentoxide, Ta2O5) formed on the anode, a solid electrolyte layer (e.g., manganese dioxide, MnO2), and a cathode. Various other layers can also be applied to the solid electrolyte layer, such as graphite and silver dispersion layers successively applied to the manganese oxide layer prior to welding the anode and cathode lead terminals onto the capacitor.
The solid electrolyte layer is generally designed to electrically connect the dielectric film and the cathode, and thus, must have a certain conductivity. In addition, the solid electrolyte layer is also designed to inhibit short-circuiting of the capacitor that results from the presence of defects in the dielectric film. For example, upon exposure to heat generated by a short-circuit current, a manganese oxide layer can be converted to an insulator and thereby inhibit further short-circuiting.
Nevertheless, despite the benefits of using manganese oxide as the solid electrolytic layer, other materials have also been utilized. For instance, some electrolytic capacitors have utilized a conductive polymer layer (e.g., polypyrrole, polythiophene, polyaniline, polyacetylene, poly-p-phenylene, and the like) as the electrolytic layer. Examples of such capacitors are described in U.S. Pat. Nos. 5,457,862 to Sakata, et al., 5,473,503 to Sakata, et al., 5,729,428 to Sakata, et al., and 5,812,367 to Kudoh, et al.
For instance, Sakata, et al. '862 describes forming a conductive polymer layer by polymerizing an aniline monomer on a dielectric oxide film using an oxidant. Sakata, et al. '862 states, however, that because such conductive layers are thin, they become damaged by thermal stress generated upon mounting the capacitor, thereby increasing leakage current. Thus, Sakata, et al. '862 also describes forming a first conductive polymer layer formed on the oxide layer and a second conductive polymer layer formed on the first conductive polymer layer.
Moreover, Sakata, et al. '428 describes a capacitor having an electron donor organic compound layer covering the dielectric oxide film and a conductive polymer layer as the solid electrolytic layer. Sakata, et al. '428 states that the electron donor layer can reduce normalized leakage current at higher temperatures when using a conductive polymer as the electrolytic layer. Examples of such electronic donor organic compounds are said to be fatty acids, aromatic carboxylic acids, anionic surface agents (carboxylate or sulfonate), phenol and derivatives thereof, silane coupling agents, titanium coupling agents, and aluminum coupling agents.
Nevertheless, despite the benefits obtained by utilizing a conductive polymer layer, various problems still remain with the capacitors formed therefrom. For instance, capacitors utilizing a conductive polymer layer still tend to short-circuit and have a relatively high equivalent series resistance (“ESR”), which refers to the extent that a capacitor acts like a resistor when charging and discharging in an electronic circuit.
As such, a need currently exists for an improved electrolytic capacitor that inhibits short-circuiting and has decreased ESR.